Appliance for improved venting

ABSTRACT

A gas appliance, flue assembly, or vent adapter including an adjustable flow regulator which regulates the proportions and volume of dilution air and combustion products into the vent is disclosed. The flow regulator can be adjusted to allow a given appliance to exhaust vent gases through a range of different venting systems constructed from a wide range of materials. The appliance installer may adjust the appliance vent gases for a particular pre-existing or installed vent. The flow regulator also provides flow resistance which helps prevent backdrafting and the free escape of dilution air (which may be heated room air in some instances) through the vent to the outside atmosphere.

This invention relates to venting systems for gas-burning appliances.More specifically, the present invention relates to a device thatadjusts the dilution air flow and combustion product flow from anappliance to adapt the appliance vent gas composition for ventingsystems built from a variety of materials.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not Applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

BACKGROUND OF THE INVENTION

Conventional gas heating appliances such as furnaces, boilers, and waterheaters provide the consumer with safe, economical space and waterheating, while requiring little maintenance during a long lifespan.These appliances typically use single wall galvanized vent connectorsand either a masonry chimney or Type B vent pipe to vent the flue gasescreated during operation. The American National Standards Institute(ANSI) categorizes gas appliances based on the pressure produced in aspecial test vent and the difference between the actual temperature anddew point temperature of the flue gas.

A category I appliance is one which has a vent expected to operate undernegative static vent pressure with a minimum of condensation. A categoryI furnace or boiler has an Annual Fuel Utilization Efficiency (AFUE)range of 78% minimum to approximately 83%. Moisture does not condensefrom the flue gas in category I appliances because the actual flue gastemperature is generally more than 140° F. above its dew pointtemperature. Traditional draft hood equipped appliances are category Iappliances. However, many mid-efficiency, fan-assisted appliances arecategory I appliances as well. Such appliances can be made category Iappliances by adjusting the flue gas temperature to be in the same rangeas the traditional category I appliance, and by designing the ventsystem to maintain a negative pressure even in the presence of the fan.Venting systems for category I appliances typically include Type Bvents, lined masonry chimneys, and single wall metal vents.

Category II appliances also operate with negative vent pressure.However, the vent gas temperature is generally less than 140° E aboveits dew point temperature. The condensation occurring in theseappliances requires the designer to use a corrosion resistant vent toexhaust the flue gases. There are few, if any, category II appliances onthe market.

Category III appliances operate with a positive vent pressure and with avent gas temperature generally at least 140° F, above its dew pointtemperature. Their AFUE ranges from approximately 78% to 83%. Becausethe pressure in the vent exceeds that of the surrounding atmosphere,these appliances require an airtight vent to prevent leakage of fluegases before they reach the outside venting location. An example of acategory III appliance would be a mid-efficiency furnace that is ventedhorizontally through the wall. Venting systems for category IIIappliances typically include high temperature plastic and single wallstainless steel metal vents.

Category IV appliances include furnaces, boilers, and other devices thatoperate with a positive vent pressure and with a vent gas temperatureless than 140° F above its dew point temperature. They generally haveAFUE values above 83%. Because the pressure in the vent exceeds that ofthe surrounding atmosphere and because condensation occurs in the vent,these appliances require an airtight, corrosion-resistant vent that isequipped for condensate disposal. Category IV appliances are usuallyhigh-efficiency, condensing devices. Venting systems for category IVappliances typically include high temperature plastic, polyvinylchloride (“PVC”), or chlorinated polyvinyl chloride (“CPVC”) vents.

ANSI Z21.47-1993 provides the current category certificationrequirements for gas furnaces. These requirements define and use theconcept of Steady State Thermal Efficiency (SSTE) in making a categorydetermination. SSTE measures the appliance's operating efficiency bydividing the total gas energy input to the appliance into the amount ofenergy gainfully used by the appliance (essentially one minus the amountof energy expelled up the flue (wasted energy)) while the appliance isoperating in a steady state. AFUE, on the other hand, is an overallassessment of an appliance's annual operating efficiency. ANSIZ21.47-1993 uses flue gas temperature and the flue gas carbon dioxidecontent to distinguish between category I and non-category I appliancesbased on a SSTE of 83%. The flue gas temperature of an appliance with agiven SSTE varies with the amount of excess air used for combustion andthe amount of dilution air added prior to the vent. These amounts, inturn, determine the percentage of carbon dioxide in the flue (7-9% formost appliances). The ANSI specification indicates, for example, that anappliance having between 7-9% carbon dioxide in the flue gas qualifiesas a category I appliance when the flue gas is approximately 140° F, ormore above its dew point temperature.

Assigning an appliance to a specific category is important because thecategory determines the type, size, material, and installationrequirements of the venting system for that specific appliance. Forexample, a category I appliance may use traditional venting materialssuch as Type B vent pipe or a masonry chimney, while a category IVfurnace requires a vent system built from corrosion resistant materials,and category III and IV appliances require airtight vent systems.

The flue gas of natural draft appliances, such as furnaces and waterheaters, contains a large amount of water vapor. As the industry hasmoved to high efficiency appliances, and subsequently to lower flue gastemperatures, condensation of water and corrosive substances from theflue gas onto exhaust conduit surfaces has become a major design issue.Most new appliances are connected to an old vent, often using a singlewall vent connector. In many cases, the vent is a masonry chimney.However, in today's building codes, the use of single wall metal ventconnector is severely limited, and most masonry chimneys requirerelining before the new appliance may be installed. Converting to a TypeB connector from a single will connector may cost the building owner upto 60.00, while relining a chimney to protect against condensation cancost from around 200 to 800. For another example, problems with categoryIII appliances using high temperature plastic vents have prompted somejurisdictions and some appliance manufacturers to prohibit the use ofhigh temperature plastics. Alternative stainless steel vent systems costroughly twice as much as high temperature plastic systems, in the 100 to300 range. In short, the existing vent may be completely inadequate forthe new appliance and may either prevent the building owner frominstalling gas appliances or require the building owner to undergo anexpensive and time consuming vent system replacement.

In an attempt to avoid these costs, several manufacturers have designedappliances with draft hoods that entrain dilution air into the vent.Entraining dilution air into the vent reduces the amount of condensationformed during operation and therefore reduces the number ofinstallations which would require chimney relining. Unfortunately, thisprocess also allows heated room air to escape in an uncontrolledfashion, both while the appliance is operating and while the applianceis idle. The escaping heat increases the heat load on the building andtherefore increases the energy cost associated with controlling thebuilding temperature. In addition, the typical draft hood equippedappliance is susceptible to backdrafting, which is especiallytroublesome in the multi-story housing market.

BRIEF SUMMARY OF THE INVENTION

Accordingly, one object of the present invention is to decrease theinstalled cost of a modern gas appliance.

Another object of the present invention is to decrease the overallenergy consumption of a building.

Still another object of the present invention is to increase theinstallation venting options available to the gas appliance installer.

Yet another object of the present invention is to reduce backdraftingand increase the safety of the appliance vent system.

One or more of the preceding objects, or one or more other objects whichwill become plain upon consideration of the present specification, aresatisfied in whole or in part by the invention described in thisspecification.

One aspect of the invention is a gas-burning appliance adapted forvarying the proportions of combustion products to dilution air in itsvent gas. The appliance can be a furnace, a water heater, a boiler, orsome other gas appliance which is externally vented and normally usedwithin a building or other structure.

A combustion chamber is provided for burning gas and producingcombustion products. A flue gas outlet is included for passing flue gasto a mixing chamber. A dilution air inlet is used for passing dilutionair into the mixing chamber. The appliance has at least one valveelement defining at least a first dilution air aperture and at least afirst combustion product aperture.

In one embodiment of the invention, the valve element is a flat plateand the apertures are pairs of holes in the plate. The differentapertures can also be formed in other ways, as by the cooperation of tworelatively movable elements (analogous to the rotating covers of somespice or parmesan cheese dispensers).

The valve element may be fixed, or the valve element may be movablebetween one or more positions. In the first position of a moveable valveelement, the first dilution air aperture is placed between the dilutionair inlet and the flue gas mixing chamber, and the first combustionproduct aperture is placed between the combustion chamber and the fluegas mixing chamber. The first dilution air and first combustion productaperture pair are respectively adapted to provide a first ratio ofdilution air to combustion products passing into the flue gas mixingchamber when the valve element is in its first position.

In the second position of a movable valve element, the second dilutionair aperture is positioned between the dilution air inlet and the fluegas mixing chamber, and the first combustion product aperture ispositioned between the combustion chamber and the flue gas mixingchamber. The second dilution air and second combustion product aperturepair are respectively adapted to provide a second ratio of dilution airto combustion products passing into the flue gas mixing chamber when themovable valve element is in its second position.

The first and second ratios of dilution air to combustion productspassing into the flue gas mixing chamber are different, due to thedifferent size hole ratios or other adaptations of the dilution airaperture and combustion product aperture. This allows thecombustion-products-to-dilution-air ratio to be selected to match theappliance to the venting system it will be attached to. This allowsmodern, high-efficiency gas appliances to be connected to traditionalventing systems without causing vent corrosion, and without producing aninappropriately high or low pressure of combustion products in the vent.

In the configuration in which the valve element is fixed, the firstdilution air aperture is placed between the dilution air inlet and theflue gas mixing chamber. The first combustion product aperture is placedbetween the combustion chamber and the flue gas mixing chamber. Thevalve element is secured in this position to continuously provide afirst ratio of dilution air to combustion products in the flue gasmixing chamber.

Another aspect of the invention is an adapter for varying theproportions of combustion products to dilution air in the vent gas of afuel-burning appliance. The adapter has a dilution air inlet: acombustion product inlet: a flue gas mixing chamber, and at least onefixed or movable valve element. The valve element defines at least afirst dilution air aperture and at least a first combustion productaperture, and has at least a first position and hole ratio as describedbefore. The adapter can be part of the appliance, part of the ventingarrangement, or a separate, add-on installation for attachment betweenan appliance and a venting arrangement.

Yet another embodiment of the invention is a flue assembly adapted forvarying the proportions of combustion products to dilution air passingthrough it The assembly comprises a dilution air inlet: a combustionproduct inlet: a flue gas mixing chamber, and at least one fixed ormovable valve element as previously defined. Again, the assemblyprovides one or more ratios of dilution air to combustion productspassing into the vent. This flue assembly can be installed in a buildingto adapt the building to receive a variety of gas appliances havingdifferent categories.

One significant advantage of the invention is its simplicity, as theflows of dilution air and combustion air can be coordinated by operatinga single valve element. The valve element or adjacent structure can bemarked to indicate the proper positions of the valve element fordifferent categories of appliances (if it is installed as part of thevent system) or different vent types (if it is incorporated in theappliance), or both. This multiple-function valve element makesselection of the proper valve element position much less subject tomiscalculations and errors, such as confusion about which of twoseparate valve elements controls the dilution air and which controls thecombustion products. A fixed valve element would not require anyadjustment in the field to obtain the correct ratio of dilution air tocombustion products.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating one embodiment of the presentinvention.

FIG. 2 shows a cross-section of a movable valve aperture plate takenalong line 2—2 in FIG. 1, and having three pairs of apertures forcategory I, III, and IV appliances.

DETAILED DESCRIPTION OF THE INVENTION

While the invention will be described in connection with one or morepreferred embodiments, it will be understood that the invention is notlimited to those embodiments. On the contrary, the invention includesall alternatives, modifications, and equivalents as may be includedwithin the spirit and scope of the appended claims.

FIG. 1 shows a gas burning appliance, for example a gas furnace,generally indicated by reference numeral 10. The appliance 10 burnsnatural gas, propane, or some other combustible gas in the combustionchamber 12 The resulting combustion product gases flow through the heatexchanger 14, the flue product passage 16, and the combustion flue inlet18 into the flue gas mixing chamber 20. The combustion flue inlet 18 isshown in FIG. 1 as one end of the flue product passage 16. The appliance10 also draws dilution air into the dilution air inlet opening 22 fromoutside the furnace 10, preferably from outside the building beingheated. The dilution air then passes through a dilution air inlet 24,and into the mixing chamber 20. The combustion gases and dilution airboth flow through the orifice plate 26, which is the valve element inthis embodiment, then they mix in the flue gas mixing chamber 20 to formvent gases (combustion products mixed with dilution air). The blower 28helps draw flue gas and dilution air through the mixing chamber 20during on-cycles and helps exhaust the vent gases through the vent 29 tothe outside atmosphere. The position of blower 28 downstream of thedilution air and combustion product inlet serves to restrict off-cycleair flows through the dilution air inlet and through the combustion flueinlet. The vent 29 may be constructed from any of the materialsappropriate for a category I, II, III, or IV appliance. The blower 28may be an integral part of the appliance, but is not so limited and mayalso be part of the vent system.

The orifice plate 26 includes first and second major faces and 31perforated by pairs of dilution air apertures and combustion productapertures that may be rotated and secured the dilution air opening 24and the combustion flue inlet 18.

FIG. 2 shows an orifice plate 26 with three pairs of apertures: CategoryI apertures 32 and 34, and category III apertures 40 and 42, category IVapertures 36 and 38. Each “aperture” as defined here may include morethan one opening within the scope of the present invention. The selectedaperture 32, 36, or 42 passes combusting gas from the combustion flueinlet 18. The selected aperture 34, 38 or 40 passes dilution air fromthe dilution air inlet 24. The orifice plate 26 may contain as many oras few aperture pairs as the size of the manufacturer's orifice plate,the combustion flue opening, and the dilution air opening allow. Theselected aperture pair 32-34. 36-38, or 40-42 controls the ratio ofdilution air and combustion product gases in the flue gas mixing chamber20 so the resulting vent gas may pass through the selected or existingvent 29 without damaging the vent 29 materials or causing undesiredcondensation. In FIG. 2, the aperture pair 32-34 have been selected byregistering them with the dilution air inlet 24 and the combustion flueinlet 18.

Each diametrically opposed aperture pair 32-34. 36-38. 40-42 mayrestrict the dilution air and flue gas flows by different amounts and indifferent ratios to configure the appliance for a different type of vent29 material. During installation of the appliance 10, the installerrotates the orifice plate 26 to its proper position based on theconstruction of the vent 29. The proper position is indicated by thecategory legends I, III, and IV, one of which is aligned with anexternal reference mark 44. The proper position places the particularpair of holes 32-34, 36-38, 40-42 which match the vent gas mixture forthe construction of vent 29 over the dilution air inlet 24 and thecombustion flue inlet 18.

The openings in the orifice plate 26 (and the blower 28, if present)generate flow resistance that makes the appliance 10 less susceptible tobackdrafting than a typical draft hood equipped appliance. The flowresistance also restricts the flow of dilution air during the appliance10 off-cycle, which helps to prevent heated air from escaping freelythrough the vent 29. Thus, less energy is required to maintain roomtemperature.

Each appliance 10 manufacturer may use different diameters or shapes forthe dilution air inlet 24 and the combustion flue inlet 18. The orificeplate 26 itself and its hole pairs 32-34, 36-38 and 40-42 are notrestricted to a round shape, but need only control the ratio of dilutionair to flue gas entering the post-orifice mixing chamber region 20. Themanufacturer uses Table 1 to determine the proper size for the orificeplate 26 hole pairs, 32-34. 36-38, and 40-42, that will appropriatelyadjust the dilution air/flue gas mixture for their desired appliancecategory. The orifice plate 26 is preferably constructed from anon-corrosive stainless steel.

Natural gas produces about 1,000 Btus of heat energy per cubic foot ofgas burned. About 14 cubic feet of air are needed per cubic foot ofnatural gas for acceptable combustion and a gas appliance with nodilution air needs to exhaust about 15 cubic feet of combustion productsper 1.000 Btu. A gas furnace that operates at 100,000 Btus per hourneeds to exhaust about 1,500 cubic feet of combustion products per houror about 22 standard cubic feet per minute (scfm). Dilution air, as usedin Table 1, is measured as a percentage of flue products. A table valueof 100 percent dilution air, for example, means approximately 15 cubicfeet of dilution air per 1000 Btu of gas burned, for a total of 30 cubicfeet of vent gases per 1,000 Btu. In other words, a hole pair in theorifice plate 26 must be sized to allow equal amounts of dilution airand combustion gas to mix in the flue gas mixing chamber 20. A gasfurnace that operates at 100,000 Btu per hour, which needs 100 percentdilution air, needs to exhaust approximately 44 scfm of vent gases. Asshown in Table 1, the percentage of dilution air required differsdepending on whether the appliance uses outdoor (42° F.) dilution air,or indoor (60° F.) dilution air.

As an example, assume that a manufacturer anticipates that its indoordilution air. SSTE 81 appliance will be installed in locations that mayhave one of three venting systems: PVC, CPVC, or high-temperatureplastic. In this situation, rather than design and manufacturing threeseparate appliances that meet the vent gas requirements for eachpossible venting system, the manufacturer may design and manufacture oneappliance with an orifice plate 26 having three aperture pairs 32 and34-36 and 38, and 42. Table 1 indicates, for example, that the orificeplate 26 should include a hole pair 36 and 38 that mixes approximately300% dilution air to combustion products for a PVC system, a hole pair32 and 34 that mixes approximately 110% dilution air to combustionproducts for a CPVC system, and a hole pair 40 and 42 that mixesapproximately 150% dilution air to combustion products for ahigh-temperature plastic vent system. The suggested mixing percentagesin Table 1 are targeted at meeting the flue gas criteria (also shown inTable 1). For example, keeping the flue gas temperature under 140° F, ina PVC vent system. Furthermore, the installer need not know beforehandwhich venting system the installation site uses, because the installercan rotate the orifice plate 26 during installation to adjust the fluegas output of the appliance 10 for the venting system used in thebuilding.

The orifice plate 26 is not limited to any particular number ofapertures or sets of apertures, nor to any particular aperture shape ornumber of apertures. The manufacturer, for example, may choose to use alarge plate with enough area for many aperture pairs, or a small platewith enough area for fewer aperture pairs. The apertures need only besized and positioned correctly to adjust the mixture of combustionproduct gases and dilution air according to Table 1. The SSTE ranges andthe flue gas criteria shown in Table 1are not all inclusive. Theinvention may be used with additional SSTE ratings or additionalcriteria not indicated in the table simply by determining the criteriaof interest and adjusting the orifice plate 26 aperture pairs such as32-34, 36-38 or 40-42 to meet those criteria.

TABLE 1 Approximate Vent Ditution Air Requiremsnts for Gas AppliancesVenting System: Plastic PVC Plastic High Temperature Vent System CPVCVent Plastic Vent System System Flue gas Flue Gas Flue Gas Vent DriesCriteria: Temperature Temperature Completety Less Than Less Than 140° F.210° F. Outdoor (42° F.) Dilution Air SSTE 80 350% 130% 200% SSTE 81300% 110% — SSTE 82 250% 80% — SSTE 83 200% 60% — Indoor (60° F.)Dilution Air SSTE 80 350% 130% 100% SSTE 81 300% 110% 150% SSTE 82 250%80% 300% SSTE 83 200% 60% — SSTE 85 100% 10% — SSTE 87 30% 0% —Approximate Vent Dilution Air Requirements for Gas Appliance VentingHigh Temperature Type B Interior Exterior System: Plastic Vent MasonryMasonry Vent System System Chimney Chimney Flue gas All InteriorMaintain Maintain Maintain Criteria: Portions of Negative NegativeNegative the Vent Pressure: Pressure: Pressure: Dry by the Avoid AvoidAvoid End of the Excessive Excessive Excessive Burner On- Conden-Conden- Conden- cycle sation sation sation Outdoor (42° F.) Dilution AirSSTE 80 100% 0% — — SSTE 81 200% 0% — — SSTE 82 — 0% — — SSTE 83 — 0% —— Indoor (60° F.) Dilution Air SSTE 80 50% 0% 50% — SSTE 81 100% 0% — —SSTE 82 150% 0% — — SSTE 83 200% 0% — — SSTE 85 — — — — SSTE 87 — — — —*Dilution air required to cool flue gases to a safe temperature isdetermined by the requirements for the warmest expected day (60° F.);condensation is based on a typical day (42° F.).

We claim:
 1. A gas-burning appliance adapted for varying the proportionsof combustion products to dilution air in its vent gas, said appliancecomprising: A. a combustion chamber for burning gas and producingcombustion products: B. a combustion flue inlet for passing flue gasfrom said combustion chamber to a mixing chamber: C. a dilution airinlet for passing dilution air into said lo mixing chamber: D. at leastone valve element defining at least first and second dilution airapertures and at least first and second combustion product apertures,said at least one valve element being movable between: i. a firstposition causing said first dilution air aperture to regulate the flowof dilution air through said dilution air inlet and causing said firstcombustion product aperture to regulate the flow of combustion productsthrough said combustion flue inlet: and ii. a second position causingsaid second dilution air aperture to regulate the flow of dilution airthrough said dilution air inlet and causing said second combustionproduct aperture to regulate the flow of combustion products throughsaid combustion flue inlet: wherein said first dilution air and firstcombustion product apertures are respectively adapted to provide a firstratio of dilution air to combustion products passing into said mixingchamber and said second dilution air and second combustion productapertures are respectively adapted to provide a second ratio of dilutionair to combustion products passing into said mixing chamber, whereinsaid first and second ratios are different.
 2. The gas-burning applianceof claim 1, wherein said at least one valve element further defines atleast a third dilution air aperture and at least a third combustionproduct aperture, said valve element has a third position causing saidthird dilution air aperture to regulate the flow of dilution air throughsaid dilution air inlet and causing said third combustion productaperture to regulate the flow of combustion products through saidcombustion flue inlet, and said third dilution air and third combustionproduct apertures are respectively adapted to provide a third ratio ofdilution air to combustion products passing into said mixing chambers,wherein said first, second, and third ratios are different.
 3. Thegas-burning appliance of claim 1, wherein said at least one valveelement has first and second major faces, and said apertures areperforations extending through said first and second major faces.
 4. Thegas-burning appliance of claim 3, wherein said at least one valveelement is a plate having said major faces disposed substantiallyparallel to each other on opposite sides of said plate.
 5. Thegas-burning appliance of claim 4, wherein said plate is rotatablesubstantially in a plane substantially parallel to said major facesbetween at least said first and second positions.
 6. The gas-burningappliance of claim 3, further comprising a combustion product passagefor passing combustion products from said combustion chamber into saidmixing chamber and a dilution air passage for passing dilution air fromsaid dilution air inlet into said mixing chamber, wherein said dilutionair passage and said combustion product passage are adjacent to saidfirst major face, and said mixing chamber is adjacent to said secondmajor face.
 7. The gas-burning appliance of claim 6, further comprisinga blower located downstream of said combustion flue inlet and saiddilution air inlet for drawing flue gas and dilution air through saidmixing chamber.
 8. The gas-burning appliance of claim 1, configured as afurnace.
 9. The gas-burning appliance of claim 8, further comprising ablower located downstream of said combustion flue inlet and saiddilution air inlet for drawing flue gas and dilution air through saidmixing chamber.
 10. The gas-burning appliance of claim 1, configured asa water heater.
 11. The gas-burning appliance of claim 10, furthercomprising a blower located downstream of said combustion flue inlet andsaid dilution air inlet for drawing flue gas and dilution air throughsaid mixing chamber.
 12. The gas-burning appliance of claim 1,configured as a boiler.
 13. The gas-burning appliance of claim 12,further comprising a blower located downstream of said combustion flueinlet and said dilution air inlet for drawing flue gas and dilution airthrough said mixing chamber.
 14. The gas-burning appliance of claim 1,further comprising a blower located downstream of said combustion flueinlet and said dilution air inlet for drawing flue gas and dilution airthrough said mixing chamber.
 15. An adapter for varying the proportionsof combustion products to dilution air in the vent gas of a fuel-burningappliance, said adapter comprising: A. a dilution air inlet: B. acombustion flue inlet: C. a mixing chamber; and D. at least one valveelement defining at least first and second dilution air apertures and atleast first and second combustion product apertures, said at least onevalve element being movable between: i. a first position causing saidfirst dilution air aperture to regulate the flow of dilution air throughsaid dilution air inlet and causing said first combustion productaperture to regulate the flow of combustion products through saidcombustion flue inlet: and ii. a second position causing said seconddilution air aperture to regulate the flow of dilution air through saiddilution air inlet and causing said second combustion product apertureto regulate the flow of combustion products through said combustion flueinlet: wherein said first dilution air and first combustion productapertures are respectively adapted to provide a first ratio of dilutionair to combustion products passing into said mixing chamber, and saidsecond dilution air and second combustion product apertures arerespectively sized to provide a second ratio of dilution air tocombustion products passing into said mixing chamber, wherein said firstand second ratios are different.
 16. The adapter of claim 15, whereinsaid at least one valve element further defines at least a thirddilution air aperture and at least a third combustion product aperture,wherein said third dilution air and third combustion product aperturesare respectively adapted to provide a third ratio of dilution air tocombustion products passing into said mixing chambers, wherein saidfirst, second, and third ratios are different.
 17. The adapter of claim15, wherein said at least one valve element has first and second majorfaces, and said apertures are perforations communicating through saidfirst and second major faces.
 18. The adapter of claim 17, wherein saidat least one valve element is a plate having said major faces disposedsubstantially parallel to each other on opposite sides of said plate.19. The adapter of claim 18, wherein said plate is rotatablesubstantially in a plane substantially parallel to said major facesbetween said first and second positions.
 20. The adapter of claim 19,wherein said dilution air inlet and said combustion product inlet areadjacent to said first major face, and said mixing chamber is adjacentto said second major face.
 21. A flue assembly adapted for varying theproportions of combustion products to dilution air passing through it,said assembly comprising: A. a dilution air inlet: B. a combustion flueinlet; C. a vent: and D. at least one valve element defining at leastfirst and second dilution air apertures and at least first and secondcombustion product apertures, said at least one valve element beingmovable between: i. a first position causing said first dilution airaperture to regulate the flow of dilution air through said dilution airinlet and causing said first combustion product aperture to regulate theflow of combustion products through said combustion flue inlet: and ii.a second position causing said second dilution air aperture to regulatethe flow of dilution air through said dilution air inlet and causingsaid first combustion product aperture to regulate the flow ofcombustion products through said combustion flue inlet: wherein saidfirst dilution air and first combustion product apertures arerespectively adapted to provide a first ratio of dilution air tocombustion products passing into said vent and said second dilution airand second combustion product apertures are respectively sized toprovide a second ratio of dilution air to combustion products passinginto said vent, wherein said first and second ratios are different. 22.The flue assembly of claim 21, wherein said at least one valve elementfurther defines at least a third dilution air aperture and at least athird combustion product aperture, wherein said third dilution air andthird combustion product apertures are respectively adapted to provide athird ratio of dilution air to combustion products passing into saidvent, wherein said first, second, and third ratios are different. 23.The flue assembly of claim 21, wherein said at least one valve elementhas first and second major faces, and said apertures are perforationscommunicating through said first and second major faces.
 24. The flueassembly of claim 23, wherein said at least one valve element is a platehaving said major faces disposed substantially parallel to each other onopposite sides of said plate.
 25. The flue assembly of claim 24, whereinsaid plate is rotatable substantially in a plane substantially parallelto said major faces between said first and second positions.
 26. Theflue assembly of claim 23, wherein said dilution air inlet and saidcombustion product inlet are adjacent to said first major face, and saidflue is adjacent to said second major face.
 27. A gas burning appliancedisposed within a building adapted for controlling the proportion ofcombustion products to dilution air in its vent gas, said appliancecomprising: A. a combustion chamber for burning gas and producingcombustion products; B. a combustion flue inlet for passing flue gasfrom said combustion chamber to a mixing chamber; C. a dilution airinlet for passing dilution air into said mixing chamber; D. at leastfirst and second flow restrictors, said first flow restrictor secured ina position regulating the flow of dilution air through said dilution airinlet and said second flow restrictor secured in a position regulatingthe flow of combustion products through said combustion flue inlet;wherein said first and second flow restrictors are defined by at leastone valve element having first and second major faces and perforationsextending through said first and second major faces, and wherein saidfirst and second flow restrictors are respectively adapted to provide aratio of dilution air to combustion products passing into said mixingchamber; and E. a low-temperature vent in fluid communication with saidmixing chamber and providing a conduit for combustion products out ofthe building.
 28. The gas-burning appliance of claim 27, wherein said atleast one valve element is a plate having said major faces disposedsubstantially parallel to each other on opposite sides of said plate.29. The gas-burning appliance of claim 27, further comprising acombustion product passage for passing combustion products from saidcombustion chamber into said mixing chamber and a dilution air passagefor passing dilution air from said dilution air inlet into said mixingchamber, wherein said dilution air passage and said combustion productpassage are adjacent to said first major face, and said mixing chamberis adjacent to said second major face.
 30. The gas-burning appliance ofclaim 29, further comprising a blower located downstream of saidcombustion flue inlet and said dilution air inlet for drawing flue gasand dilution air through said mixing chamber.
 31. The gas-burningappliance of claim 27, configured as a furnace.
 32. The gas-burningappliance of claim 31, further comprising a blower located downstream ofsaid combustion flue inlet and said dilution air inlet for drawing fluegas and dilution air through said mixing chamber.
 33. The gas-burningappliance of claim 27, configured as a water heater.
 34. The gas-burningappliance of claim 33, further comprising a blower located downstream ofsaid combustion flue inlet and said dilution air inlet for drawing fluegas and dilution air through said mixing chamber.
 35. The gas-burningappliance of claim 27, configured as a boiler.
 36. The gas-burningappliance of claim 35, further comprising a blower located downstream ofsaid combustion flue inlet and said dilution air inlet for drawing fluegas and dilution air through said mixing chamber.
 37. The gas-burningappliance of claim 27, further comprising a blower located downstream ofsaid combustion flue inlet and said dilution air inlet for drawing fluegas and dilution air through said mixing chamber.
 38. The gas burningappliance of claim 27 wherein said low-temperature vent is designed tohold pressure that exceeds atmospheric pressure.
 39. The gas burningappliance of claim 27 wherein the low-temperature vent comprises one ofa high temperature plastic, polyvinyl chloride, and chlorinatedpolyvinyl chloride.
 40. The gas burning appliance of claim 27 wherein ablower is positioned within said low-temperature vent.
 41. The gasburning appliance of claim 27 wherein a blower is positioned betweensaid low-temperature vent and said mixing chamber.